The optical pickup device (also referred to as an optical head, an optical head device, or the like) for replaying and recording the information on a light information recording medium (also referred to as an optical disk, or a medium) such as a CD (a compact disk), DVD (a digital video disk, or a digital versatile disk) have been developed and produced, and thus popularized.
Recently, the standard of the information recording medium which enabled the higher density information recording has been researched and developed.
Such optical pickup device forms a spot by collecting beam of light emitted from mainly a laser diode as a light source through an optical system including an optical component such as a beam shaping prism, a collimator, a beam splitter, an objective optical component, or the like, onto the information recording face of an optical disc, next collects reflection from an information recording hole (also called as a pit) on the recording face through a same optical system onto a sensor at this time, and then converts into an electrical signal, to playback the information. During this time, ‘0’ and ‘1’ information are discriminated based on the phenomenon that the light beam of reflectance varies according to the shape of the information recording hole. On the information recording face of an optical disc, a protective layer made of plastic, which is also called as a cover glass, is provided as a protective substrate.
When recording information on recording type media such as CD-R, CD-RW, and the like, a spot resulted from the laser beam is formed on a recording surface and a thermochemical change is generated in a recording material on the recording face. Accordingly, for example in the case of CD-R, the thermal diffusivity pigment is irreversibly changed and a shape same to the information recording hole is formed. In the case of CD-RW, since a phase change-type material is used, a reversible change between a crystalline state and non-crystalline state by the thermochemical change is generated, and thus the rewriting of the information is possible.
For the optical pickup device for playing back the information from an optical disc of a CD standard, the numerical aperture (NA) of an objective lens is around 0.45, and the wavelength of a light source for use is around 785 nm. In addition, as for the recording, ones having 0.50 in approximate is a lot used. Herein, the thickness of a protective substrate for an optical disc of the CD standard is 1.2 mm.
A CD has been widely popularized as an optical information recording medium, and for the last few years, a DVD is popularized. The DVD is increased in its information recording capacity by making the thickness of the protective substrate thinner than the CD and also by miniaturizing the information recording hole. While a recording capacity of a CD is about 600 to 700 MB (Mega Bite), a DVD has a large recording capacity of about 4.7 GB (Giga Byte), thus being used a lot as a distribution medium to which a moving image such as a movie picture is recorded.
In addition, the optical pickup device for playing back the information from an optical disc of a DVD standard is principally the same as that of the CD. However, since the information recording hole is miniaturized as described above, the optical pickup device employs an objective lens having the NA of around 0.60, and a light source having the wavelength of around 655 nm. Further, as for the recording, ones having 0.65 in approximate is more often used. Herein, the thickness of the protective substrate for an optical disc of the DVD standard is 0.6 mm.
A recording type for the optical disc of the DVD standard is already put to practical use, and there are various standards such as DVD-RAM, DVD-RW/R, DVD+RW/R, and the like. The technical principal of these optical disc is also same as that of the CD standard.
As described above, there has been proposed an optical disc of high density/high capacity. This optical disc is to use the light source for providing the light having a wavelength of around 405 nm, which is the light source for providing so-called a blue-violet laser.
For such ‘optical disc of high density’, even if the wavelength to be used is determined, the thickness of the protective substrate, recording capacity, NA, and the like cannot be determined in a single uniform way.
If a way to improve the recording density substantially is selected, the thickness of the protective substrate of an optical disc is reduced, and accordingly the NA is increased. Alternatively, the thickness of the protective substrate and NA can be in the same standard as the conventional optical disc standard. At this time, the physical recording density is not significantly increased, but the properties required as the optical system become relatively gradual. In specific, there is proposed a protective substrate such as further reduced ones having the thickness of 0.1 mm, or ones same with DVD of 0.6 mm.
The optical component to be used in the above-described optical pickup device is mostly formed by an injection molding with a plastic resin or pressure molding with a glass. Of these, the glass-made optical component is generally small in the refractive-index variance to a temperature change. Therefore, this element can be used in a beam shaping prism disposed nearby a light source which is the heat source. However there is a problem that the manufacturing cost is high. Therefore, it is less employed in each of optical components of collimator, coupling lens, objective optical component, and the like. On the other hand, the plastic resin-made optical component has a merit that the manufacturing cost is low as it is molded by injection, and thus is used a lot to a large extent. However, since the plastic material has an absorption in the wavelength area to various degrees or another, there is a problem that the optical properties for a use are deteriorated.
Hence, in order to perform a playing back of information (reading) or recording of information at high speed, it is necessary to improve the light amount to surely form a spot of collected light. For this, a most simple method is to increase the light emitting amount of the diode by raising the power of a laser diode, but due to this if optical properties involved in the use are increasingly deteriorated, a problem arises in that the optical properties as designed cannot be attained. In addition, increase in an atmospheric temperature due to the raise of the laser power becomes a factor that promotes a deterioration of the resin.
Further, if the operation is carried out at high speed, the actuator also operates at high speed, and thus generated heat also becomes a factor that promotes a deterioration of the resin. Accordingly, there is proposed various efforts to control the change of the optical properties at the time of use.
For example, in Patent Document 1, there is disclosed a technique of blending 0.03 to 1 parts by mass of a hindered amine light-resistant stabilizer, 0.002 to 2 parts by mass of phenol antioxidant, and 0.002 to 1 parts by mass of phosphorous antioxidant, based on 100 parts by mass of a thermoplastic norbornene resin (for example, a hydroxide additive of ring-opening polymer of 1,4-methano-1,4,4a,9a-tetrahydrofluorene). However, the stability for light of a resin composition disclosed in Patent Document 1 is not sufficient, and thus is not appropriate to be used in an optical pickup device having the blue-violet laser light source. In addition, there is a flaw in transmittance that it is lowered due to a coloring as salt is formed by the phenol antioxidant and the hindered amine light-resistant stabilizer. There are also problems that the foaming at the time of molding is easy to occur, and since the birefringence is poor, an optical component of high density cannot be obtained.
Also, for example in Patent Document 2, there is disclosed a technique to obtain a resin composition excellent in a processing stability, light-resistant stability, heat resistance, and transparency, by containing a vinyl alicyclic hydrocarbon polymer and a hindered amine light-resistant stabilizer having the number average molecular weight (Mn) of 1,000 to 10,000. According to the method, the foaming at the time of molding and the birefringence are improved as compared to the above-described technique, but still the stability for light is insufficient, and thus is not appropriate to be used in an optical pickup device having the blue-violet laser light source. In addition, this method has a flaw in that white turbidity occurs due to the blue-violet laser light irradiation.
Further, in Patent Document 3, as the resin composition having excellent weather resistance, light resistance, transparency, heat resistance, and a low dusting characteristic at the time of molding process, and exhibiting excellent optical properties when molded to an optical component, there is disclosed a weather-resistant resin characterized by containing (A) a cyclic polyolefin-based resin, (B) benzotriazole-based UV absorbent having the molecular weight of 300 or more, the vapor pressure at 20° C. temperature of 1×10−8 Pa or less, and the 5% weight reducing temperature with a heat loss measurement of 200° C. or above, and (C) a hindered amine light stabilizer having the molecular weight of 500 or more, the vapor pressure at 20° C. temperature of 1×10−6 Pa or less, and the 5% weight reducing temperature with a heat loss measurement of 250° C. or above. According to the method, the heat resistance is improved and the foaming at the time of molding is controlled as in the above-described technique, but there is absorption with the benzotriazole ultraviolet absorber, and thus is not appropriate to be used in an optical pickup device having the blue-violet laser light source. In addition, there is a flaw that the water absorption is high.
In Patent Document 4, in order to obtain a molded product with no coloration and no color change although irradiated with UV ray for a long period, there is disclosed a technique of mixing pellet A formed from a resin composition containing 100 parts by mass of a vinyl alicyclic hydrocarbon polymer and 0.001 to 2.0 parts by mass of an antioxidant with pellet B formed from a resin composition containing 100 parts by mass of a vinyl alicyclic hydrocarbon polymer and 2 to 20 parts by mass of a light-resistant stabilizer at a ratio of 5≦A/B≦50 by mass, and then melt-molding the resultant. However, the stability at the time of molding is deteriorated, and the transparency of the resin and the stability for the light are both insufficient, thus is not appropriate to be practically used in an optical pickup device using the blue-violet laser light source. In addition, the method is inappropriate for a large-scale production as the manufacturing and molding processes are complicated.
In the Patent Document 5, there is disclosed a resin composition comprising a polymer (A) which is produced by subjecting an aromatic vinyl monomer to an addition polymerization reaction, and then hydrogenating an aromatic ring, and an antioxidant (B) having a phosphate ester structure and a phenol structure in a molecule such as 6-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetrakis-t-butyldibenzo[d,f][1.3.2]dioxaphosphepin. It is described that the molded product of the resin composition is excellent in mechanical strength, and is not colored even with the irradiation of a light beam such as a blue-violet laser with a short wavelength and a high strength. However, the optical properties are still not sufficiently stable due to a deterioration of the resin during the use. Therefore, it is in fact difficult to use this resin composition in the optical pickup device having the blue-violet laser light source.
[Patent Document 1] Japanese Unexamined Patent Publication No. H09-268250
[Patent Document 2] WO 01/092412
[Patent Document 3] Japanese Unexamined Patent Publication No. 2001-72839
[Patent Document 4] Japanese Unexamined Patent Publication No. 2003-276047
[Patent Document 5] Japanese Unexamined Patent Publication No. 2004-83813